The method herein disclosed is used to determine mine roof competency, i.e., whether a mine ceiling is likely to collapse, by detecting and surveying cracks in rock strata through testing with packer assemblies in boreholes drilled in the mine ceiling.
Other methods of asserting mine roof competency are known. For example, many mine roof warning devices detect minor shifts in the strata of the mine ceiling which indicate impending collapse. Most of the devices that sense these shifts have had only limited success due to their complexity and/or insensitivity. Further, all of these devices will only provide a warning after the ceiling has already reached an extremely dangerous state. By the time these devices alert the mine personnel to the danger it often is too late to adequately prepare for the collapse. The need has hence been recognized for a system that detects mine roof flaws well before the time when the roof collapses.
More sophisticated techniques such as spectral analysis of acoustical impact signals, ultrasonic reflectivity, and infrared temperature differential analysis have been suggested as means for evaluating mine roof competency, but all are limited to the detection of the first crack above the mine ceiling. A truly effective technique must be able to determine whether there are additional cracks located above the lowest crack.
Inflatable packers have been used in mine boreholes for purposes unrelated to determining mine roof competency. Packers are inflatable bladders that are used to seal off vertical sections of the borehole so that pressurized fluid can be supplied to an isolated portion of the borehole. U.S. Pat. No. 4,072,015 discloses the use of a packer assembly to supply pressurized fluid between inflated packers to prevent the collapse of a borehole. U.S. Pat. No. 4,474,409 discloses a packer assembly in which a fracturing fluid is supplied through a conduit to an isolation zone in a borehole between two packers. The fluid pressure is increased until the surrounding rock is fractured. The purpose of this induced fracturing is to allow methane gas to be released from pockets in coal mines. This patent also mentions that the packer assembly can be used to measure variations in permeability along the length of the borehole by injecting pressurized fluid. The fluid used in that method is supplied from the mine working area and comprises water and other materials, often sand. There is no indication in the reference that the disclosed packer can be used to assess the likelihood of mine roof collapse.